1. Field of the Invention
The present invention relates to a jar can for a secondary battery, and more particularly, it relates to a jar can for a secondary battery which is lightweight and attains high rigidity.
2. Description of the Prior Art
Following the remarkable spread of portable telephones and portable electronic apparatuses in recent years, miniaturization and weight reduction of such apparatuses are increasingly required. These apparatuses require miniature and lightweight sealed batteries having high storage capacity. Thus, thin angular batteries having prismatic shapes are increasingly required in place of conventional cylindrical batteries. The sealed batteries include a nickel-hydrogen storage battery, a lithium ion secondary battery and the like, for example.
A first exemplary conventional jar can employed for a nickel-hydrogen storage battery is now described with reference to FIGS. 5A to 5C. Referring to FIGS. 5A to 5C, a bottomed battery jar can (jar can) 13 of a nickel-hydrogen storage battery 1 has an open upper end. This jar can 13 stores or contains a plate group 16 including a positive electrode 7, a positive lead tab 5, a negative electrode 11 and a separator 9. A valve element 4, a cap 2, a rivet 8, an insulating plate gasket 10, a washer 12 and a frame body 14 are provided above the plate group 16. A lid body 6 is provided on the upper end opening of the jar can 13, for electrically isolating the jar can 13 from the cap 2 through the insulating plate gasket 10.
The jar can 13 is generally prepared from a nickel-plated steel sheet. In order to cope with recent weight reduction of apparatuses or the like, a jar can prepared from aluminum or an aluminum alloy has been developed.
A second exemplary conventional jar can for an angular sealed battery disclosed in Japanese Patent Laying-Open No. 6-52842 (1994) is now described. Referring to FIG. 6, a bottomed angular jar can 51 has wider and narrower side surfaces 52 and 55 and an open upper end.
This jar can 51 has been provided in order to suppress deformation resulting from increase in internal pressure, in particular. Therefore, the wider side surfaces 52 are larger in thickness than the narrower side surfaces 55.
However, the aforementioned conventional jar cans 13 and 51 have the following problems:
The first exemplary jar can 13 shown in FIGS. 5B and 5C contains a positive electrode active material, a negative electrode active material and an electrolyte. Pressure is applied from the interior to the side and bottom surfaces of the jar can 13 due to generation of gas resulting from reaction between the active materials and the electrolyte or electrochemical reaction in charging of the battery 1 and swelling of the electrodes 7 and 11.
At this time, the wider side surfaces of the jar can 13 are more readily deformed by the internal pressure as compared with the narrower side surfaces. Therefore, the width of each narrower side surface of the jar can 13 increases from t.sub.0 to t in a portion causing swelling deformation of each wider side surface, as shown in FIG. 7.
In general, an aluminum killed steel sheet (SPCE material under Japanese Industrial Standards) is employed as a nickel-plated steel sheet which is applied to a jar can. This SPCE material has a relatively large Young's modulus of about 20000 kgf/mm.sup.2. Therefore, the aforementioned swelling deformation can be suppressed in a specific range by setting the thickness of the SPCE material at about 0.4 mm.
In this case, however, the jar can weighs about 7 g since the specific gravity of the SPCE material is about 7.8. Consequently, the jar can makes up about 30 to 40% of the total weight of a battery to which the same is applied, leading to a problem in weight.
When a SPCE material having a smaller thickness is applied in order to reduce the weight of the jar can, however, its rigidity is so reduced that the aforementioned swelling deformation exceeds the specific range. Thus, reduction of the thickness of the SPCE material is so limited that the weight of the jar can cannot be readily reduced.
On the other hand, the Young's modulus of a light metal such as aluminum or an aluminum alloy is about 7000 kgf/mm.sup.2. This value is about 1/3 of that of the SPCE material. When ajar can is prepared from aluminum or the like in the same thickness as the SPCE material, therefore, swelling deformation of the jar can further increases to exceed the specific range.
When the thickness of the aluminum material is increased in order to increase its strength, however, the weight of the jar can is also increased. If the thickness is increased while keeping the outer dimensions unchanged, further, the volume in the jar can is reduced. Thus, the capacity of a battery to which the jar can is applied may be reduced.
To this end, a material having a high Young's modulus is studied in relation to various materials of aluminum and aluminum alloys. In the present circumstances, however, a material having a high Young's modulus is inferior in moldability for deep drawing or the like and cannot provide a practical jar can.
The second exemplary jar can 51 has been proposed in order to suppress swelling deformation, as described above. In case of molding such a jar can provided with wider and narrower side surfaces having different thicknesses by deep drawing in general, however, material sheets having portions of different thicknesses corresponding to the wider and narrower side surfaces are required. Therefore, the cost for the material sheets is so increased that the jar can cannot be economically obtained.